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.TH PTHREAD_ATFORK 3C "Dec 12, 2003"
.SH NAME
pthread_atfork \- register fork handlers
.SH SYNOPSIS
.LP
.nf
#include <sys/types.h>
#include <unistd.h>

\fBint\fR \fBpthread_atfork\fR(\fBvoid (*\fR\fIprepare\fR) (void), \fBvoid (*\fR\fIparent\fR) (void),
     \fBvoid (*\fR\fIchild\fR) (void));
.fi

.SH DESCRIPTION
.sp
.LP
The \fBpthread_atfork()\fR function declares fork handlers to be called prior
to and following \fBfork\fR(2), within the thread that called \fBfork()\fR. The
order of calls to \fBpthread_atfork()\fR is significant.
.sp
.LP
Before \fBfork()\fR processing begins, the \fIprepare\fR fork handler is
called. The \fIprepare\fR handler is not called if its address is \fINULL.\fR
.sp
.LP
The \fIparent\fR fork handler is called after \fBfork()\fR processing finishes
in the parent process, and the \fIchild\fR fork handler is called after
\fBfork()\fR processing finishes in the child process. If the address of
\fIparent\fR or \fIchild\fR is  \fINULL\fR, then its handler is not called.
.sp
.LP
The \fIprepare\fR fork handler is called in  \fBLIFO\fR (last-in first-out)
order, whereas the \fIparent\fR and \fIchild\fR fork handlers are called in
\fBFIFO\fR (first-in first-out) order. This calling order allows applications
to preserve locking order.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBpthread_atfork()\fR returns \fB0\fR. Otherwise,
an error number is returned.
.SH ERRORS
.sp
.LP
The \fBpthread_atfork()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBENOMEM\fR \fR
.ad
.RS 11n
Insufficient table space exists to record the fork handler addresses.
.RE

.SH USAGE
.sp
.LP
Solaris threads do not offer \fBpthread_atfork()\fR functionality (there is no
\fBthr_atfork()\fR interface). However, a Solaris threads application can call
\fBpthread_atfork()\fR to ensure \fBfork()\fR-safety, since the two thread APIs
are interoperable. See \fBfork\fR(2) for information relating to \fBfork()\fR in
a Solaris threads environment in Solaris 10 relative to previous releases.
.SH EXAMPLES
.LP
\fBExample 1 \fRMake a library safe with respect to \fBfork()\fR.
.sp
.LP
All multithreaded applications that call \fBfork()\fR in a POSIX threads
program and do more than simply call \fBexec\fR(2) in the child of the fork
need to ensure that the child is protected from deadlock.

.sp
.LP
Since the "fork-one" model results in duplicating only the thread that called
\fBfork()\fR, it is possible that at the time of the call another thread in the
parent owns a lock. This thread is not duplicated in the child, so no thread
will unlock this lock in the child.  Deadlock occurs if the single thread in
the child needs this lock.

.sp
.LP
The problem is more serious with locks in libraries.  Since a library writer
does not know if the application using the library calls \fBfork()\fR, the
library must protect itself from such a deadlock scenario.  If the application
that links with this library calls \fBfork()\fR and does not call  \fBexec()\fR
in the child, and if it needs a library lock that may be held by some other
thread in the parent that is inside the library at the time of the fork, the
application deadlocks inside the library.

.sp
.LP
The following describes how to make a library  safe with respect to
\fBfork()\fR by using  \fBpthread_atfork()\fR.

.RS +4
.TP
1.
Identify all locks used by the library (for example \fB{L1,\|.\|.\|.Ln}\fR).
Identify also the locking order for these locks (for example
\fB{L1\|.\|.\|.Ln}, as well\fR.)
.RE
.RS +4
.TP
2.
Add a call to \fBpthread_atfork(f1, f2, f3)\fR in the  library's
\fI\&.init\fR section.  \fBf1\fR, \fBf2\fR, \fBf3\fR are defined as follows:
.RE
.sp
.in +2
.nf
\fB        f1(\|)
        {
                /* ordered in lock order */
                pthread_mutex_lock(L1);
                pthread_mutex_lock(\|.\|.\|.);
                pthread_mutex_lock(Ln);
        }

        f2(\|)
        {
                pthread_mutex_unlock(L1);
                pthread_mutex_unlock(\|.\|.\|.);
                pthread_mutex_unlock(Ln);
        }

        f3(\|)
        {
                pthread_mutex_unlock(L1);
                pthread_mutex_unlock(\|.\|.\|.);
                pthread_mutex_unlock(Ln);
        }\fR
.fi
.in -2

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.sp
.LP
.BR exec (2),
.BR fork (2),
.BR atexit (3C),
.BR attributes (7),
.BR standards (7)
